This invention relates to fluid bed dryers and more particularly to a method and apparatus for salvaging product which is otherwise wasted in a fluid bed dryer utilizing a rotating air distribution disc to deliver rotating streams of air to the fluid bed.
Fluid bed dryers are used for drying particulate material by passing streams of fluidizing gases through a bed of material in a so-called fluidizing chamber. Fluid bed dryer operation involves passing sufficient air or other fluid through the chamber to suspend solid particles in a stream of air. If each particle is bathed with a stream of air to produce the so-called "fluid bed", rapid drying throughout the entire mass is achieved.
The fluidizing chambers are frequently cylindrical in form and have perforate or screen-like bottom portions which have holes sufficiently small to retain the particulate material while permitting air or other fluid to flow therethrough. The selection of a hole size requires a compromise between a small hole size which would retain nearly all of the particulate material being dried but unduly restrict air flow and a large hole size which would allow less restricted air flow but allow considerable material to fall from the chamber. This compromise usually results in the selection of a hole size which allows adequate air flow while retaining a large majority of the material but allowing very small particles of the material to fall through the screen.
Of particular interest to the present invention is an improved fluid bed dryer as disclosed in U.S. Pat. No. 3,849,900. The improved fluid bed dryer of this patent is characterized by the use of a rotating apertured disc positioned below and out of contact with the product to be dried. The disc delivers a stream or streams of air moving in rotary fashion throughout the fluidizing chamber. This improved drying apparatus eliminates localized heavy streams of air which tend to develop in other prior art fluidizing chambers and avoids mechanical agitation and contact with the particles which is undesirable in many instances where particles tend to crush, smear, or agglomerate, e.g., in the case of cheese particles.
Although the rotating disc dryer is a notable improvement over prior art fluid bed dryers, it does cause increased amounts of product to fall through the screen-like bottom portion of the fluidizing chamber. This product is generally lost since it is difficult to recover and is often spoiled due to contamination at the bottom of the dryer.